[Info-vax] ODS-1 Index File extension headers

[Lee Gleason]

"Robert A. Brooks"  wrote in message news:qqurg1$p1g$1 at dont-email.me...


>Andy Goldstein says . . .

>The ODS-1 index file needs to become multi-header when it gets large. 
>Problem is ODS-1 map pointers have only one format, and they’re limited to 
>256 blocks. So one header can only map a fairly >limited size index file. I 
>don’t remember exactly how big the ODS-1 map area is, but it must be in the 
>neighborhood of 100 map pointers, so you can map about 25K blocks with one 
>header. That means it >takes 3 headers to map the ODS-1 architectural limit 
>of 65K files.

>The reason the index file extension headers are always file numbers 6 and 7 
>is that they’re pre-allocated, when the volume is initialized. This is to 
>avoid a “you can’t get there from here” problem: When >you need to extend 
>the index file, it’s because you’re out of file header blocks. But to 
>create an extension header you need another header block, and you just gave 
>away the last one with the previous >file creation. So they’re 
>pre-allocated, so you’ll have one when you need it.

>So the answers to the questions are, it’s always 6 and 7 - they’re the next 
>reserved file IDs past the original set of 5. And it’s never more than 3 
>headers because of the architectural limit of 65K files. I >don’t know 
>anything about the HOM command. Sounds from the reference like a utility 
>that does structure fixups on ODS-1 that the file system itself can’t do.

  Great! Thanks for the help. This simplifies things for me a great deal. 
Please forward my thanks along to Mr. Goldstein.

--
Lee K. Gleason N5ZMR
Control-G Consultants
lee.gleason at comcast.net